1. Field of the Invention
The present invention relates to a cathode material for a lithium secondary battery using a lithium composite oxide as a cathode active material, a lithium secondary battery, and a secondary battery module using the battery.
2. Description of Related Art
A lithium secondary battery using a lithium ion has a higher energy density per volume and weight than other secondary batteries due to a large ionization tendency and small atomic weight of lithium. Accordingly, the lithium secondary battery is widely used as a power source of portable consumer equipment such as a cell phone, a notebook-sized personal computer, and PDA (personal digital assistant).
Further, in future, such a lithium secondary battery is expected to be used as a power source for a large sized application, for example, a motor driven electric automobile capable of suppressing a CO2 emission under the environmental consideration, a hybrid vehicle driven by a motor and an engine, and a power storage system of reproducible energies such as solar light and wind power generators.
As mentioned above, such a large sized lithium secondary battery is strongly demanded to be more inexpensive and have a longer life cycle than a power source of general consumer equipment.
Currently, lithium cobalt oxide (LiCoO2) is mainly used as a cathode material of the lithium secondary battery. However, lithium cobalt oxide is produced using cobalt as a material of which production quantity is small, resulting in a high price thereof. Accordingly, it is difficult to lower the cost of the battery when lithium cobalt oxide is used therein. Further, when the battery using lithium cobalt oxide is kept under high voltage conditions, cobalt dissolves from lithium cobalt oxide, thereby to cause a remarkable decrease in the battery life cycle and relate to an environmental regulation (for example, toxicity).
In view of the above, the use of nickel, iron and manganese is investigated as a substitute metal of cobalt in order to lower the cost of a battery and elongate the life cycle thereof. For example, lithium nickel oxide (LiNiO2) using nickel has advantages excellent in being produced at a lower cost with a higher capacity and a superior toxicity/safety profile than lithium cobalt oxide. However, a cathode material using LiNiO2 has an inferior safety profile at overcharging and is remarkable decreasing the capacity during a charge/discharge cycle.
Hereby, it is possible to improve a safety profile of a cathode material by partially substituting nickel with other elements, while the safety and capacity profiles of such a cathode material are still insufficient compared to the cathode material using lithium cobalt oxide. Accordingly, the above mentioned cathode material with a substitute element is not suitable for the large sized application.
Recently, it has been paid much attention to the use of ion as lithium iron oxide (LiFePO4) with an orthorhombic olivine structure. Herein, LiFePO4 has advantages that it can be produced at a low cost and excellent in a safety profile, while the conductivity thereof is very low compared to that of LiCoO2. Further, the operation voltage of 3.4V thereof is 0.2 to 0.6V lower than that of other cathode materials, which decreases the energy density thereof.
In contrast, when a lithium manganese composite oxide is used, the price of a manganese compound used as a material is 10% or less of that of a cobalt compound, allowing the production at a low cost. Further, the electron conductivity thereof is about 10 times higher than that of lithium cobalt oxide. Accordingly, a low energy loss and a long life cycle of the lithium manganese composite oxide can be expected due to the excellent conductivity thereof.
Further, when the lithium manganese composite oxide is used, the amount of oxygen released in heat generation of a battery is decreased compared to the amount of oxygen when lithium cobalt oxide is used, which is excellent in the safety profile. Therefore, the lithium manganese composite oxide is expected to be a very promising material for producing a large sized lithium secondary battery.
However, when the lithium manganese composite oxide is used in a battery, manganese dissolves into an electrolyte solution when kept at a high temperature. The dissolved manganese may cause clogging of a separator arranged between a cathode and an anode, or form a coating film of a manganese compound on the anode. This may increase a resistance of the battery and decrease a durability profile thereof, resulting in the most important factor for improving the manganese Spinel type cathode.
Herein, the manganese dissolution from the lithium manganese composite oxide is caused because of the Jahn-Teller effect on trivalent manganese (that is, a non-symmetrical structure is more stable).
Here, the lithium manganese composite oxide is represented by LiMn2O4 as a stoichiometric composition. Since the valence of Li is 1 and the valence of O is −2, respectively, the average valence of Mn is 3.5 to satisfy the electrically neutral condition. Hereby, it is considered that about 50% of manganese in the particle of the composite oxide exists as trivalent manganese. Trivalent manganese is converted to more stable bivalent manganese and tetravalent manganese when trivalent manganese becomes energetically unstable, which causes the dissolution of bivalent manganese as an ion form. Therefore, in order to prevent the manganese dissolution, it is needed to decrease the rate of trivalent manganese, or avoid conditions in which trivalent manganese becomes energetically unstable. Herein, it is considered that manganese in a cathode material contacts to an acid component such as hydrogen fluoride (HF) contained in the electrolyte solution in which the cathode is immersed. This contact may make trivalent manganese energetically unstable.
Here, Japanese Patent Publication No. 3142522 describes that a cycle profile is improved by substituting a part of manganese sites of LiMn2O4 having a Spinel structure with lithium or transition metal in order to decrease the rate of trivalent manganese contained in a cathode material. When a part of manganese is substituted by the element having the valence of 3 or less, the average valence of manganese contained in the cathode material increases to satisfy the electric neutral condition, resulting in a decrease of the rate of the trivalent manganese. However, it is impossible to control the valence of manganese near the surface of the cathode only by substituting the element. Thus, such a substitution is insufficient to suppress the manganese dissolution from the surface of the cathode material. Further, the increase of the average valence of manganese decreases the capacity.
According to Japanese Patent Publication No. 3944899 and Japanese Laid-Open Patent Publication No. 2008-536285, a coating compound is arranged on the surface of a lithium manganese composite oxide so as to prevent a cathode material from contacting to an acid component contained in an electrolyte solution, whereby the manganese dissolution is suppressed.
Further, Japanese Patent Publication No. 3944899 discloses that a lithium manganese composite oxide is coated with a metal oxide or a metal sulfide. However, this treatment is insufficient to improve the durability profile because the lower conductivity of the coating material than the lithium manganese composite oxide increases the resistance of the coated composite oxide.
On the other hand, Japanese Laid-Open Patent Publication No. 2008-536285 discloses that a lithium compound is deposited on the surface of lithium nickel cobalt manganese oxide. However, when the lithium compound is deposited discretely, manganese dissolves from an area where no lithium compound is deposited. Accordingly, the treatment is insufficient to suppress the manganese dissolution. When a metal oxide is further coated on the surface on which a lithium compound is deposited, this coating may decrease the conductivity because the resistance also increases along with the coating.